Sudden cardiac arrest (SCA) is a major cause of death in the United States. Ventricular fibrillation (VF) represents the final common pathway for death in most patients who experience out-of-hospital SCA. Despite advances in defibrillation and resuscitation, the overall survival rate in this setting remains dismally low (-5%). Yet, the precise mechanisms and events leading to failure of resuscitation and death are poorly understood. One potential mechanism is related to a coupling between mitochondrial energetics and cardiac excitability. Depolarization of mitochondrial inner membrane potential (??m) can cause shortening of the action potential and eventual loss of excitability due to switching from mitochondrial net ATP synthesis to ATP hydrolysis, causing activation of sarcolemmal ATP-sensitive potassium (IK-ATP) channel and a massive increase in outward current. Importantly, this mechanism is modulated by the function of anaerobic glycolysis, because IK-ATP channels are preferentially controlled by glycolytically derived ATP. However, the interplay between anaerobic glycolysis, maintenance of ??m and the action potential (AP) in the setting of VF and ischemia has not been studied. The overall hypothesis of this proposal is that a combination of VF and global ischemia (VF/ischemia) act synergistically to promote mitochondrial depolarization and consequent decrease in excitability, defibrillation failure or asystole (lack of electrical activity). Furthermore, the dynamics of mitochondrial depolarization depends on the magnitude of intracellular calcium ([Ca2+]i) accumulation (Ca overload) secondary to VF/ischemia and maintenance of anaerobic glycolysis throughout the time frame relevant to sudden cardiac arrest (~10 minutes). To test these hypotheses, I propose to investigate a link between the dynamics of the AP, [Ca2+]i, and ??m during VF/ischemia in isolated blood-perfused porcine hearts. In addition, I will investigate the role of glycogen that is a key energy substrate available for ATP production in ischemic hearts. The amount of ATP remaining in the tissue will be also measured using biochemical assay. The main approach will be confocal imaging using combination of fluorescent dyes in whole hearts. The Specific Aims are:1) To investigate the relationship between ??m, [Ca2+]i dynamics, and the action potential in the whole heart subjected to ischemia/VF and defibrillation/reperfusion, 2) To investigate the role of glycogen content in the incidence of asystole during VF/ischemia and defibrillation/reperfusion. I believe that these studies will provide better understanding of the mechanisms and treatment of cardiac arrest in the setting of VF/ischemia.